1. Field of the Invention
The present invention relates to an air-fuel ratio control system for an internal combustion engine and, particularly, to a control system for controlling an air-fuel ratio of an air-fuel mixture supplied to the internal combustion engine provided with a catalyst in the exhaust system for purifying the exhaust gases.
2. Description of the Related Art
Japanese Patent Laid-open No. H05-195842 (JP-'842) discloses an air-fuel ratio control system for an internal combustion engine provided with a catalyst in the exhaust system. According to this disclosure, an actual oxygen charged degree of the catalyst is compared with a target oxygen charged degree, and the air-fuel ratio is controlled to be richer when the actual oxygen charged degree is greater than the target oxygen charged degree, while the air-fuel ratio is controlled to be leaner when the actual oxygen charged degree is less than the target oxygen charged degree. This control makes it possible to always maintain an appropriate oxygen charged degree to obtain good exhaust characteristics.
In the above conventional system, the actual oxygen charged degree is calculated according to the intake air flow rate of the engine and an output of the air-fuel ratio sensor provided upstream of the catalyst. Accordingly, the calculated value of the actual oxygen charged degree is influenced by differences or aging in the detection characteristics of the air-fuel ratio sensor. In order to reduce such an influence, JP-'842 discloses a method for calculating an average value of an output of an air-fuel ratio sensor provided downstream of the catalyst and an average value of the output of the air-fuel ratio sensor provided upstream of the catalyst and correcting the output of the upstream air-fuel ratio sensor according to a ratio of the two average values.
However, the oxygen concentration of exhaust gases in the vicinity of each of these air-fuel ratio sensors may become different from each other due, for example, to a change in the engine operating condition, to the configuration of the exhaust pipe, or to the mounted positions of the upstream air-fuel ratio sensor and downstream air-fuel ratio sensor. The difference in oxygen concentrations may cause a change in the mutual relationship between the outputs of the two air-fuel ratio sensors in a short time period.
For example, in a high-load operating condition of the engine, the velocity of exhaust gases increases, and the oxygen concentration in the exhaust flow shown by the solid line A1 in FIG. 17 may slightly deviate from the oxygen concentration in the exhaust flow shown by the broken line A2. In this example, the upstream air-fuel ratio sensor 101 detects an oxygen concentration in the exhaust flow shown by the solid line A1. On the other hand, on the downstream side of the catalyst, the exhaust flows shown by the solid line A1 and the broken line A2 on the upstream side are mixed to create the exhaust flow shown by the thick solid line A3. Thus, the downstream air-fuel ratio sensor detects an oxygen concentration in the exhaust flow (A3). Therefore, the outputs of the two sensors differ from each other even if the detection characteristics of the two air-fuel ratio sensors 101 and 102 are completely identical.
The method disclosed in JP-'842 cannot appropriately deal with the deviation in the outputs of two sensors that is caused in a short time period due to the change in the engine operating condition.